In U.S. Pat. No. 4,471,449 issued on Sept. 11, 1984 to Steven C. Leavitt et al, entitled "Scan Conversion System", and assigned to Hewlett-Packard Company, apparatus is described for deriving from analog signals produced along each of a plurality of radial lines in a sector data values that are to be displayed at pixels at rectilinear coordinates along parallel raster lines of a cathode ray tube. After two adjacent radial lines of raw data have been received and data is just beginning to be received along a third radial line, a state machine defines a step-by-step path through all of the pixels located in the slice between the radial lines. The steps are one pixel in length and go along the X or Y axis as required. By starting from a pixel location having a known radius and angular position with respect to the radial lines and iteratively adding certain values at each step, the radius and angular position of each pixel can be determined. The analog data along the radial lines is sampled and means utilizing the radial position of each pixel as it is attained selects four samples of raw data. A first intermediate interpolated data value is derived at the radius of the pixel from the two samples on one radial line, and a second interpolated data value is derived at the radius of the pixel from the two data samples on the other radial line. A final data value for display at each pixel is derived by interpolating the first and second interpolated data values. This process is carried out for every pixel in each slice in the sector.
Whereas it works very well, the path of pixels for each slice is different and is effectively stored in memory associated with the state machine. If it should be desired to expand a given portion of the sector, a feature normally referred to as "zoom", new paths through the pixels of each slice containing a portion of the selected area would have to be provided for each degree of magnification. A large amount of memory would be required for each degree of magnification, and it would not be practicable to provide continuous zoom from one magnification to the other.